Turn back the clock just a little: make userboot.so always be 4th

Turns out there was a hidden dependency we hasn't counted upon. The
host load /boot/userboot.so to boot the VMs it runs. This means that
the change to lua meant suddently that nobody could run their older
VMs because LUA wasn't in 10.0, last month's HardenedBSD, 11.2 or
whatever. Even more than for the /boot/loader* binaries, we need a
good coexistance strategy for this. While that's being designed and
implemented, drop back to always 4th for userboot.so. This will fail
safe in all but the most extreme environments (but lua-only hacks
to .lua files won't be processes in VMs until we fix it).

Differential Review: https://reviews.freebsd.org/D16805